Triple quadrupole mass spectrometer systems capable of performing MS/MS usually have two precision quadrupole mass spectrometers separated by a RF-only quadrupole which is operated as a gas collision cell. The first mass spectrometer ("Q1") is used to select a specific ion mass-to-charge ratio (m/z), and to transmit ions with that m/z ratio into the RF-only quadrupole or collision cell ("Q2"). The selected ions (also referred to as the parent ions) are accelerated to an energy of several tens of electron volts before entering the collision cell.
In the RF-only quadrupole collision cell, some or all of the parent ions are fragmented by collisions with a background gas (which is commonly argon or nitrogen added to the collision cell at a pressure of up to several millitorr). The fragment ions, along with any unfragmented parent ions, are transmitted through Q2 into the second precision quadrupole ("Q3"), which is operated in a mass resolving mode. The mass resolving mode of Q3 is normally either to scan over a specified mass range, or else to transmit selected ion fragments by peak hopping (i.e. by being rapidly adjusted to select specific ion m/z ratios in sequence). The ions which are transmitted through Q3 are detected by an ion detector, the signal from which is registered by a data system.
Triple quadrupoles of the kind described are known to be very sensitive and very specific analytical instruments. The sensitivity is due in part to the efficient transmission of ions through the quadrupoles, and to the efficient confinement of ions in the RF-only collision cell. The high specificity is due to the specific nature of the combination of mass selection by Q1, fragmentation to create characteristic fragments in Q2, and mass selection of the fragments in Q3.
Operation of a triple quadrupole as described above requires that the mass resolving quadrupoles Q1 and Q3 operate in a high vacuum region (less than 10.sup.-5 torr), while the collision cell Q2 operates at a pressure of up to several millitorr. Efficient transfer of ions in and out of Q2 requires that the entrance and exit apertures of Q2 be as large as possible. However this results in the need for large vacuum pumps in order to pump the gas which leaks from Q2 into the vacuum chambers containing Q1 and Q3.
In addition, many modern triple quadrupole systems are used with atmospheric pressure ionization sources, such as electrospray, or APCI (atmospheric pressure chemical ionization). The ions which are created in the ion source at atmospheric pressure must be sampled into the vacuum chamber through a small orifice. The gas which enters the vacuum chamber along with the ions to be analyzed, imposes another load on the vacuum pump system, typically of an amount similar to that imposed by the gas leaking from the collision cell.
In one typical configuration now on the market, ions which enter from an APCI or electrospray source are focussed through an RF-only quadrupole which is in front of Q1. This RF quadrupole ("Q0") acts as an efficient containment device for ions and transmits the ions efficiently into Q1. Thus the entire system may contain up to four quadrupoles, in which Q0 and Q2 are RF-only, both operating at a pressure of a few millitorr, while Q1 and Q3 are mass resolving, both operating at a pressure of approximately 10.sup.-5 torr.
The configuration described above requires high capacity and costly pumps. While the configuration described above can be operated as a single mass spectrometer (with no collision gas in Q2, and Q3 in an RF-only mode), rather than as an MS/MS system, it still requires substantial pumping capability.
A known related device, the quadrupole ion trap mass spectrometer, can also provide single MS and MS/MS capabilities. The quadrupole ion trap operates at a pressure of about 1 millitorr of helium, and both mass separation and fragmentation are performed in the same region of space, separated in time. Thus the ion trap is a sequential-in-time device, while the triple quadrupole is a sequential-in-space device.